The present invention relates to the fields of optical and magneto-optical recording. More particularly, it relates to improvements in focus and/or tracking servo systems for maintaining a beam of radiation in sharp focus and/or at a desired location on a moving recording element (e.g. an optical or magneto-optical disk).
In the fields of optical and magneto-optical recording, many different schemes have been proposed and utilized for controlling the focus quality and tracking accuracy of a write/read laser beam on a spinning optical or magneto-optical disk. Such schemes typically employ a closely spaced and equi-sized pair (sometimes two pair) of photodetectors arranged to be irradiated by the beam after the beam has been reflected (or transmitted) by the disk. A lens serves to focus the reflected beam upon the detector pair and, depending upon the direction of mis-focus or mis-tracking of the write/read beam on the disk, one of the photodetectors is more exposed to the reflected beam than the other, i.e., a larger area of one of the detectors is irradiated. The respective outputs of the photodetectors are then subtracted (e.g. by a conventional differential amplifier) to derive an error signal which, for example, can be used to control the focal position of the objective lens used to initially focus the write/read beam on the disk. Such a servo system continuously strives to refocus or reposition the beam to minimize the error signal.
In conventional optical recording systems, the intensity of the beam reflected (or transmitted) by the recording element may vary over a range of 30 to 1. Such intensity variations result, in large part, from predictable changes in laser power, as occasioned by switching the optical head power between write and read modes. But sizable and non-predictable variations in reflected beam intensity can also result, for example, from optical noise in the laser cavity and unintentional reflectivity variations in the recording element. These intensity variations adversely effect the stability of the focus and tracking servo systems and, to the extent possible, should be normalized out of the photodetector output.